

...where did you read that? The label states it is safe for use with fish and planted tanks.
The only precaution I saw was, "Do not use in aquariums with invertebrates or crustaceans e.g. crabs, shrimps, or crayfish."
I had Clown Loaches, Bala Sharks, Silver Dollars and Oscars and they did not appear to be affected. The product worked for me within hours and I did lots of frequent water changes afterwards to remove as much as possible of the product from the water.
If you are worried about it's effect on the fish and you have tried all other methods (Block all sunlight, reduce feeding, frequent large water changes etc), then I think your only other option is to buy a UV.